In organic chemistry, pentadiene is any hydrocarbon with an open chain of five carbons, connected by two single bonds and two double bonds. All those compounds have the same molecular formula C5H8. The inventory of pentadienes include:1,2-pentadiene, or ethyl allene, H2C=C=CH−CH2−CH3. It and 2,3-pentadiene are the least common isomers of pentadiene. 1,3-pentadiene, H2C=CH−CH=CH−CH3 with two isomers: cis-1,3-pentadiene. trans-1,3-pentadiene, also known as piperylene. 1,4-pentadiene, H2C=CH−CH2−CH=CH2. 2,3-pentadiene, H3C−CH=C=CH−CH3, with two enantiomers (R and S). It and 1,2-pentadiene are the least common isomers of pentadiene.